1. Field of the Invention
This patent relates to ink dispensing cartridges for automated dispensing systems, such as those used for lithographic printing presses. More specifically, this patent relates to an improved plunger for an ink-dispensing cartridge.
2. Description of the Related Art
Lithography is a printmaking process in which ink is applied to a plate having both image and non-image areas. The image areas are ink-receptive and water-repellent. The non-image areas are water-receptive and ink-repellent. In rotary type presses the plate is mounted on a cylinder that rotates during printing. In one typical configuration, the plate cylinder picks up the ink at the image areas and transfers the image to a blanket cylinder, which in turn transfers the image to the paper. In multi-color sheet-fed presses, up to ten inking stations can be placed in series. Each station has its own ink feeding system and handles a separate color. As the paper sheet moves from station to station, a new color is put down at each station.
Because lithographic ink is thixotropic and very viscous, conventional lithographic ink feeding systems require a complex system of drums, vibrators and fountain rollers. In a typical lithographic ink feeding system, workers remove lithographic ink from a drum (or, in some cases, smaller tins) with specially made spatulas and spread the ink across a tray (the ink fountain).
Storing lithographic ink in drums or tins can result in wasted ink if the entire drum or tin is not used because the ink is prone to oxidation and spoilage. Thus, in recent years, smaller cartridge-type ink dispensers have been developed for use in automated ink dispensing systems. These new ink cartridges can dispense ink by two different methods. The first (automated) method is with the ink cartridge mounted over the ink fountain or the fountain roller. The ink cartridge moves back and forth across the fountain, dispensing ink into the fountain or directly onto an ink form roller to provide an even consistent layer of ink. The second (manual) method is by the use of a xe2x80x9chandgun.xe2x80x9d The ink cartridge is placed inside the handgun. The dispensing end of the cartridge is placed over the rollers and ink is dispensed when the trigger is depressed. Ink is directed into the fountain manually. With either method, because the ink is dispensed directly from the cartridge, oxidation and spoilage are reduced.
Typical lithographic ink cartridges, such as that disclosed in U.S. Pat. No. 6,192,797, incorporated herein by reference, comprise a hollow cylindrical body, a plunger and a dispensing fitment. The cylindrical body holds a supply of extrudable, highly viscous ink and has a dispensing end and a plunger end. The plunger end is sealed by the plunger which moves within the cylindrical body to extrude the ink when the plunger is forced toward the dispensing end by, for example, pneumatic pressure. The dispensing end is sealed with the dispensing fitment, which typically includes a nozzle for directing the flow of the ink.
Lithographic ink cartridges typically are filled by setting the cartridge, with the dispensing fitment attached, underneath an ink flow. The cartridges typically are filled to a predetermined weight, and then the plunger is inserted into the plunger end of the cartridge.
A disadvantage of conventional ink cartridges is that, during the ink filling process, air can get trapped between the plunger and the ink when the plunger is inserted into the cartridge and pressed against the ink. Air entrapment can cause the ink to xe2x80x9cskin overxe2x80x9d, creating a skin of partly oxidized ink on top of the ink. This skin can plug the nozzle in the dispensing fitment, or cause printing xe2x80x9chickiesxe2x80x9d, i.e., areas on a sheet without ink coverage.
Thus the primary object of the present invention is to provide a plunger that prevents air entrapment between the plunger and the surface of the ink.
Another object of the present invention is to provide a plunger and dispensing fitment that nest together to minimize unused ink.
Further and additional objects will appear from the description, accompanying drawings, and appended claims.
The present invention is an improved plunger for an ink cartridge such as those used with automated lithographic printing presses. The plunger has a bottom portion and a sidewall extending upward from the periphery of the bottom portion. The bottom portion has a positive radius. That is, when inserted into an ink-filled cartridge, the side of the bottom portion facing the ink is convex. The bottom portion also has a textured surface to define channels through which air can flow when the plunger comes into contact with the ink. The plunger""s positive radius and textured surface facilitate the movement of air from the center of the plunger toward the cartridge body, thus minimizing air entrapment between the plunger and the ink.